1. Field of the Invention
The present invention relates to liquid crystal devices for adjusting the transmission of light, a method of driving such devices, and especially an illuminating apparatus incorporating such a liquid crystal device for adjusting illumination light for television or movie filming and photographing, or for a projecting type television receiver, a slide projector or the like.
2. Description of the Related Art
A liquid crystal device using materials such as twisted nematic liquid crystal, super twisted nematic liquid crystal and ferro-electric liquid crystal needs a polarizer which causes a loss of more than 50% of unpolarized light. When a liquid crystal device using any of the above-described materials is used as a light-adjusting element in an illumination apparatus using a high power light source it is inevitable that the polarizer's temperature will be greatly increased due to absorbance of light.
In contrast, a polarizer is not needed in a liquid crystal device having a composite film in which a liquid crystal material fills a series of pores in a film that forms a transparent matrix having a three-dimensional network structure, or in which a liquid crystal material is dispersed in particles in a film that forms a transparent matrix sandwiched between transparent substrates having a pair of transparent conductive films. Therefore, the above-described disadvantage regarding heating of the polarizer is overcome.
In the above-described liquid crystal device, when no voltage is applied, the liquid crystal molecules are in a random state based on the configuration of the interface between the liquid crystal molecules and the transparent matrix, which is known as the anchoring effect. Thus, in that state incident light is scattered and the composite film is opaque. When a voltage, usually a rectangular wave or a sinusoidal wave of about 200 Hz, is applied to the region between the transparent substrates having the pair of transparent conductive films with the composite film held therebetween, the liquid crystal molecules having positive dielectric anisotropy (.DELTA..epsilon.) are oriented in the direction of the electric field and are thereby gradually ordered in a manner increasing light transmittance. Thus, an electro-optic effect is brought about, and a transparent state results. Note that the term "transmittance" generally indicates the ratio of the power of the light emitted from an element, relative to the power of the light incident to the element. However, in the case of a light-scattering type liquid crystal device, the term indicates the ratio of the power of the light emitted in the range of a certain angle, relative to the power of a collimated light incident to the element. The angle may be determined depending upon the condition for using the element. Herein, light transmitted within the range of the angle is referred to as non-scattered light.
In a conventional liquid crystal device using any materials, however, the dependence of the birefringence of the liquid crystal on wavelength basically causes the spectrum of transmitted light to change depending upon the applied voltage state. Furthermore, the spectrum of transmitted light largely fluctuates particularly in a middle state between the opaque state and the transparent state.
Also in the above-described liquid crystal device using the composite film, the relation between the light transmittance in each wavelength and the applied voltage is not constant in a middle state between the opaque state and the transparent state. Moreover, the transmittance at the same voltage greatly fluctuates depending upon the wavelength. Particularly, the light transmittance is greater for longer wavelengths than for shorter wavelengths. Therefore the spectrum of transmitted light greatly fluctuates depending upon the applied voltage. Furthermore, the ratio of transmittance of each wavelength changes depending upon the applied voltage, and therefore the color tone of the transmitted light also changes as the applied voltage changes.
Through a study for the cause of the above described effects, the following has been discovered. Namely, when a voltage is applied to a liquid crystal device the liquid crystal molecules are oriented in the direction of the electric field as described above. However, the intensity of the electric field is insufficient in the middle state between the opaque state and the transparent state, and therefore the orientation of the liquid crystal molecules is disturbed by the above described anchoring effect in the vicinity of the interface of the liquid crystal molecules and the transparent matrix. Accordingly, light of a short wavelength is mainly scattered in a region in the vicinity of the interface, the transmittance of short wavelength light is lower than the transmittance of long wavelength light, and the transmitted light gives a spectrum in which the longer wavelengths prevail.
The dependence of the scattering intensity on the wavelength in turn depends upon the degree of disturbance of the orientation of the liquid crystal molecules, based on the area of the region in which the orientation of the liquid crystal molecules is disturbed, the degree of disturbance of the liquid crystal molecules in the above-described region or the like. The degree of disturbance of the orientation depends on both the applied voltage and the anchoring effect, and therefore the transmittance of each wavelength changes depending upon the applied voltage, thus changing the color tone of the transmitted light.
Therefore, when the above-described liquid crystal device is used as a light adjusting element, it is impossible to obtain light with its shorter wavelengths prevailing, or light without any wavelength dependence i.e. white light, and the color tone of the light cannot be made constant.
Therefore, the above-described liquid crystal device has already been reduced to practice for a display with a function of switching between two states, namely opaque and transparent states, but has not yet been applied to a light-adjusting element capable of sequentially adjusting its light transmittance though such a device has long been desired.
Furthermore, the conventional liquid crystal device is not capable of operating in a high temperature environment of 100.degree. C. or higher.